I don't think you should add people to your blacklist just because they couldn't redeem or didn't redeem the gift within 48 hours. It seems kind of like your a dick saying it. I've blacklisted people over not commenting but that's only on high level giveaways (9, 10), whitelist, and group ones. I feel it's a mandatory thing on those types of giveaways since it is a lot better chances at winning.

But the not redeeming right away is a little much. I'd give them at least their 7 days they are allowed per sg and then go from there. It's all up to you though and it is your giveaway so you have every right to blacklist who you want.